The present invention relates to an exposure apparatus, and more particularly to an exposure apparatus for printing a mask pattern on a work applied with a photoresist.
Generally, in such an exposure apparatus, there are a contact type exposure apparatus with the mask for a pattern being disposed to come in contact with the work, a close type exposure apparatus with the mask being spaced in proximity to the work, and a projection type exposure apparatus in which an image of the mask pattern is projected on the work. The present invention concerns the close type exposure apparatus, and is to provide the apparatus having a proximity gap between the work and the mask and suitable for production of the work having a large exposure surface and requiring the pattern accuracy of 5-10 .mu.m.
FIG. 1 schematically shows a construction of a prior art close type exposure apparatus. A mask 2 is disposed above a surface of a work 1 applied with a photoresist with a predetermined distance D therebetween. The pattern of the mask 2 is projected on the work 1 by a light beam 3 for exposure to be printed on the work 1. Numeral 4 denotes a light source of a far ultraviolet arc lamp including mercury vapor therein (hereinafter called as mercury lamp). The light emitted from the light source is reflected by a pair of oval concave mirrors 5 to be condensed on a first cold mirror 6. The light beam 3 for exposure reaches the mirror 6 and only the ultraviolet rays necessary for exposure are reflected by the mirror 6. The reflected rays pass through a fly eye lens 7 to be reflected by a second cold mirror 6' and are collimated by a lens 8. The collimated light beam 3 is directed on the mask 2. The fly-eye lens 7 functions to uniform the brightness of the light beam 3 over the whole area of the exposure surface. A proximity gap D is disposed between the work 1 and the mask 2 so that the mask 2 does not bring in contact with the surface of the work 1 applied with the photoresist. The light source 4 uses a xenon mercury lamp of which emitted light contains bright line spectra of 365 nm, 400 nm and 435 nm. The bright lines are partially arranged in phase. It is recognized that the bright lines contain coherent light by 1-5% in all the emitted rays of light.
FIG. 2 shows the enlarged work 1 and mask 2. The mask 2 is formed with a slit or cut 9 forming a pattern. Assuming that the width of the cut 9 is l, the light beam 3 is diffracted at an edge of the mask to produce Fresnel diffraction fringes on the photoresisted surface of the work 1. Accordingly, when the geometric distance X from the position just under the edge changes, the bright line of 365 nm, for example, produces fringes with regard to the distance X. Assuming that the wavelength of the light beam 3 is .lambda., the proximity gap D, and the distance from just under the edge X, the condition in which the fringes are produced is as follows:
When .sqroot.D.sup.2 +X.sup.2 -D=m.lambda., bright fringes are produced at X.div..sqroot.2mD.lambda..
When .sqroot.D.sup.2 +X.sup.2 -D=.lambda./2(2m+1), dark fringes are produced at X.div..sqroot.2(m+1/2)D.lambda.. where m is a natural number: 1, 2, 3 . . . . The above equations are approximated using D&gt;&gt;.lambda. (when the gap D is about 400 .mu.m, .lambda. is about 0.4 .mu.m.).
The fringes are produced at the positions expressed by the above equations. When a two-dimensional pattern of the mask 2, as shown in FIG. 3(a) for example, is printed on the work 1, the printed pattern on the work 1 has distortion at the peripheral portion, specifically corner portions thereof as shown in FIG. 3(b). Since the light beam 3 for exposure is not a beam of perfect coherent light but includes about 1-5% of coherent light in the bright line, it adversely affects the reproducibility and the resolution of the printed pattern of the mask 2 on the work 1 in the pattern reproduction or printing process.
On the other hand, an exposure apparatus using electron rays or X-rays as a light source in order to eliminate the coherence in the light beam for exposure and an exposure apparatus utilizing an aplanatic projection optical system have been developed. However, such exposure apparatuses require a high degree electronic circuit for controlling a complicated lens system and a device thereof, and are expensive consequently.